The present invention relates to an operational amplifier circuit, and more particularly to an improved front end stage of an operational amplifier having increased transconductance gain and improved slew rate characteristics.
Generally, the output current of the front end stage corresponds to a hyperbolic tangent with the input differential voltage. However, for input voltages greater than a certain magnitude, slew rate limiting occurs with the output current being equal in magnitude to that of the current sources for the front end stage, thereby resulting in no further increase in output current with increases in input differential voltages. Additionally, the maximum slew rate at which the output is responsive to the input is limited in order to assure amplifier stability and minimum requirements in transconductance gain. Although various techniques have been developed to either extend the range of the output current beyond the slew rate limit or to improve the slew rate and output capabilities, these techniques result in adversely affecting other related characteristics of the amplifier such as transconductance gain, offset voltage, power dissipation and temperature coefficients.
According, it is an object of this invention to provide an improved amplifier with an improved slew rate without adversely effecting the other characteristics of the front end stage of the amplifier.
It is further an object of this invention to provide an improved amplifier with high gain capabilities.
It is another object to provide a new and useful technique for increasing both the slew rate and current output capabilities of the front end stage without adversely effecting other related characteristics of the amplifier.
It is still another object to provide an improved amplifier having an increased region over which the output current of the transconductance stage of the amplifier is responsive to a differential input voltage.
It is still a further object to provide an improved amplifier having increased power efficiency without sacrificing slew rate or amplifier gain.
These and other objects are attained by providing a large signal or slew enhancement stage connected in parallel to a small signal or front end stage of an operational amplifier, thereby improving both the slew rate and transconductance gain of the amplifier while also providing the capability of extending the linear dynamic range of the front end stage output.
The small signal stage includes at least one differential transistor pair, with each pair having a differential input and a single output. The large signal stage includes a complementary set of differential transistor pairs, a pair of active loads and corresponding output stages. The large signal stage further includes means for setting the large signal stage transconductance and current gain. Additionally, the large signal stage includes a means for biasing the stage such that it remains electrically decoupled from the front end stage until the differential input voltage reaches a threshold set by the circuit designer.
Depending upon the polarity of the input differential voltage, the small signal stage functions as either a high gain current source or current sink for a subsequent stage of the amplifier. Prior to the small signal stage reaching a selected threshold voltage, such as its slew rate limit, the large signal stage will be decoupled from the small signal stage such that no additional current is provided to the front end stage by the large signal stage. Upon the small signal stage reaching the selected threshold, however, one of the complementary sections of the large signal stage will either serve as an additional current source or current sink to the output of the front end stage. Which complementary section will conduct depends upon the polarity of the differential voltage applied to the parallel inputs of the differential transistor pairs. The threshold voltage at which the large signal stage begins to deliver output current is set by the circuit designer and will usually be set to equal the point at which the small signal stage begins to slew rate limit.
Because the large signal stage is decoupled during small signal operation, thereby providing slewing current only when needed in large signal operation, the small signal characteristics such as the settling time are not adversely affected by the large signal stage. Additionally, by adjusting the transconductance and current gain parameters of the large signal stage, the parallel stage can be made to emphasize extending the linear dynamic range of the front end stage output, or providing a fast slew rate with power efficiency.
These and other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.